The present invention relates to a thermal transfer sheet and, more particularly, to a thermal transfer sheet suitable for formation of a color image on a paper sheet having a low surface smoothness (hereinafter referred to as "rough paper sheet").
When a conventional thermal transfer sheet having a thermally-transferable color ink layer formed on a foundation and containing a wax as a principal component of a vehicle thereof is used for thermal transfer printing on a rough paper sheet, transferred color ink adheres only onto crests of the microscopically undulated surface of the rough paper sheet, but not onto troughs thereof. Therefore, an obtained print image often suffers from a defect or a void.
To overcome this problem, Japanese Unexamined Patent Publication No. 61-225090 (1986) proposes a thermal transfer sheet having a filling ink layer and a thermally-transferable color ink layer formed side by side on a foundation. The filling ink layer is first transferred onto a rough paper sheet to smooth the rough surface of the paper sheet, and then the color ink layer is transferred onto the smoothed surface to form a print image.
Where the color ink layer is transferred onto the filling ink layer preliminarily transferred on the paper sheet to form a print image, however, the gloss of the print image is frequently reduced with the lapse of time.
As a result of an intensive study to identify the cause of the time-related reduction in the gloss of the print image, the present inventors have found that, although a paraffin wax is widely used as a material for the filling ink layer to improve the transferability, the use of the paraffin wax reduces the gloss of the print image with the lapse of time.
In view of the foregoing, it is an object of the present invention to provide a thermal transfer sheet which can prevent the time-related reduction in the gloss of the print image even with a filling ink layer containing a paraffin wax.
The foregoing and other objects of the present invention will be apparent from the following detailed description.